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Spherical Earth
The concept of a spherical Earth dates back to around the 6th century BC, when it was mentioned in ancient Greek philosophy, but remained a matter of philosophical speculation until the 3rd century BC, when Hellenistic astronomy established the spherical shape of the earth as a physical given. The paradigm was gradually adopted throughout the Old World during Late Antiquity and the Middle Ages.〔Continuation into Roman and medieval thought: Reinhard Krüger: "(Materialien und Dokumente zur mittelalterlichen Erdkugeltheorie von der Spätantike bis zur Kolumbusfahrt (1492) )"〕〔Direct adoption of the Greek concept by Islam: Ragep, F. Jamil: "Astronomy", in: Krämer, Gudrun (ed.) et al.: ''Encyclopaedia of Islam'', THREE, Brill 2010, without page numbers〕〔Direct adoption by India: D. Pingree: "History of Mathematical Astronomy in India", ''Dictionary of Scientific Biography'', Vol. 15 (1978), pp. 533−633 (554f.); Glick, Thomas F., Livesey, Steven John, Wallis, Faith (eds.): "Medieval Science, Technology, and Medicine: An Encyclopedia", Routledge, New York 2005, ISBN 0-415-96930-1, p. 463〕〔Adoption by China via European science: Jean-Claude Martzloff, “Space and Time in Chinese Texts of Astronomy and of Mathematical Astronomy in the Seventeenth and Eighteenth Centuries”, ''Chinese Science'' 11 (1993–94): 66–92 (69) and Christopher Cullen, "A Chinese Eratosthenes of the Flat Earth: A Study of a Fragment of Cosmology in Huai Nan tzu 淮 南 子", ''Bulletin of the School of Oriental and African Studies'', Vol. 39, No. 1 (1976), pp. 106–127 (107)〕 A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's expedition's circumnavigation (1519−1522).〔Pigafetta, Antonio (1906). Magellan's Voyage around the World. Arthur A. Clark. ()〕
The concept of a spherical Earth displaced earlier beliefs in a flat Earth: In early Mesopotamian mythology, the world was portrayed as a flat disk floating in the ocean and surrounded by a spherical sky, and this forms the premise for early world maps like those of Anaximander and Hecataeus of Miletus. Other speculations on the shape of Earth include a seven-layered ziggurat or cosmic mountain, alluded to in the Avesta and ancient Persian writings (see seven climes).
The realization that the figure of the Earth is more accurately described as an ellipsoid dates to the 18th century (Maupertuis).
In the early 19th century, the flattening of the earth ellipsoid was determined to be of the order of 1/300 (Delambre, Everest). The modern value as determined by the US DoD World Geodetic System since the 1960s is close to 1/298.25.〔See figure of the Earth and Earth radius for details. Recent measurements from satellites suggest that the Earth is, in fact, slightly pear-shaped. Hugh Thurston, ''Early Astronomy'', (New York: Springer-Verlag), p. 119. ISBN 0-387-94107-X.〕
==History==
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